The present invention relates in general to insulated gate bipolar transistors, and more particularly, to an insulated gate bipolar transistor with enhanced ON resistance in switching performance.
Recently, a class of power semiconductor devices, which is a hybrid of metal oxide semiconductor (MOS) and bipolar power technologies, has evolved. These hybrid devices are commonly known as insulated gate bipolar transistors (IGBTs) or insulate gate transistors (IGTs). IGBTs combine low drive current advantage and fast turn-on times of power MOS devices with the high current and high voltage characteristics of bipolar devices. Because of their superior characteristics for low and medium frequency applications, when compared with bipolar transistors and power MOSFETs, a substantial amount of work has been undertaken to obtain a good understanding of the operating characteristics. A vigorous effort has been underway in the industry to improve power ratings by increasing both current and voltage handling capability. Further, an intense effort has been directed at reducing turn-off time of IGBTs which has severely limited their use in high frequency applications.
A conventional IGBT comprises a power MOS structure such as a diffused MOS (DMOS) device formed in an N epitaxial layer. In a conventional MOS device, the N epitaxial layer is formed on an N-type substrate. In contrast, the N epitaxial layer formed on a P+ substrate for an IGBT. Although slight modifications must be made to the device elements formed in the epitaxial layer, the primary difference between the conventional MOS and an IGBT is that an IGBT uses a P-type substrate rather than an N-type substrate. The P-type substrate injects minority carriers into the N epitaxial layer when the IGBT is in forward conduction mode, thereby increasing conductivity in the N-type epitaxial region.
One disadvantage of conventional IGBT structures is that minority carriers which are injected into the N epitaxial region must recombine before conduction in the device can be halted. Because the N epitaxial layer is lightly doped, particularly for high voltage devices, and relatively thick, the recombination time before the minority carriers are annihilated is long. This results in a long turn-off time for the IGBT.
Another disadvantage of the conventional IGBT structure is that the conductivity modulation effect is not noticeable until the PN junction formed by the N epitaxial and the P-type substrate becomes forward biased. Because little current flows in the IGBT before this PN junction is forward biased, low current conductivity is significantly lower than a conventional MOS device. Thus while the IGBT is superior for high current switching applications, it is inferior at low current switching or in applications where both low and high current switching are required.
A conventional MOS devices comprises an integral drain source diode which allows reverse current to flow through the device when the device is turned off. In many power switching applications such as totem pole and H-bridge motor switching circuits, the integral drain source diode is quite useful. Indeed, without the integral drain source diode an additional high current diode is required external to the device. Until now, however, the integral drain source diode of a conventional IGBT has been unavailable because the PN junction formed by the N-type epitaxial layer and the P-type substrate blocks the integral diode. Because of this, IGBTs required additional components which can more than double the cost of a comparable MOS device.
Accordingly, it is an object of the present invention is to provide an insulated gate bipolar transistor with enhanced low current conductivity.
Another object of the present invention is to provide an IGBT having reduced turn-off time and improved switching time.
A further object of the present invention is to provide an IGBT having a useable integral drain source diode.
A further object of the present invention is to provide a method of manufacturing the semiconductor substrate for use with an enhanced IGBT.
Still a further object of the present invention is to provide a method for manufacturing an enhanced IGBT.